1. Field of the Invention
The present invention relates to a control apparatus for controlling engine rotational speed of a cylinder-injection spark-ignition internal combustion engine capable of injecting fuel in the compression and intake strokes.
2. Description of the Related Art
In order to reduce harmful exhaust gas components or improve fuel efficiency in a spark-ignition internal combustion engine mounted on a vehicle, there have recently been proposed various types of cylinder-injection gasoline engines, in which fuel is injected directly into the combustion chamber, in place of conventional manifold-injection engines.
A cylinder-injection gasoline engine is designed so that an air-fuel mixture with an air-fuel ratio near the stoichiometric air-fuel ratio is produced around a spark plug at the time of ignition, for example, by injecting the fuel from a fuel injection valve into a cavity formed on the top of a piston, thereby enabling ignition with a generally lean air-fuel ratio. Accordingly, the discharge amounts of carbon monoxide (CO) and hydrocarbon (HC) can be reduced, and the fuel efficiency during idle operation or low-load running can be improved considerably.
In a gasoline engine of this type, the fuel injection mode is switched between a compression stroke injection mode (late injection mode) and an intake stroke injection mode (early injection mode). Thereby, during low-load operation, the fuel is injected in the compression stroke, so that the air-fuel mixture with an air-fuel ratio near the stoichiometric air-fuel ratio can be formed around the spark plug or in the cavity. As a result, satisfactory ignition is enabled even with a generally lean air-fuel ratio. During medium- and high-load operation, the fuel is injected in the intake stroke, so that the air-fuel mixture with a uniform air-fuel ratio can be formed in the combustion chamber. As a result, like the manifold-injection engine, the output required in acceleration or high-speed running can be secured by burning large amounts of fuel.
In this cylinder-injection gasoline engine as well, like the conventional manifold-injection internal combustion engine, the idle speed control is carried out to maintain a stable idle speed in idle operation.
In this case, when the fuel injection mode is the intake stroke injection mode and the engine is in the idle operation state, the fuel is injected in the intake stroke, so that the idle speed control can be carried out as with the case of the manifold-injection engine.
In the case where an external load of accessories such as an air conditioner is applied or released, the engine speed Ne is decreased or increased. In the intake stroke injection mode, to prevent the decrease or increase, the intake air quantity is corrected by the opening control of an air bypass valve (ABV), which functions mainly as an idle speed controller. Further, to increase or decrease the engine output with high response, the advance angle correction or lag angle correction of ignition timing is made temporarily immediately after the application or release of external load, thereby rapidly stabilizing the idle speed.
However, in the cylinder-injection gasoline engine, in case the fuel injection mode is the compression stroke injection mode, ignition is effected when fuel spray is collected satisfactorily in the vicinity of the spark plug, thereby achieving good combustion (MBT (minimum advance for best torque) setting). When the fuel injection mode is the compression stroke injection mode and the engine is in the idle operation mode, therefore, although the correction of intake air quantity can be made, the deterioration in combustion may be caused if the correction of ignition timing is made to improve the response. Therefore, it is not preferable to make ignition timing correction for idle speed control in the compression stroke injection mode.